Autoinjector with retracting needle

ABSTRACT

An automatic drug delivery device is described, comprising: a housing ( 1 ); a skin sensor element ( 15 ) coupled to the housing and movable relative to the housing, wherein the skin sensor element is biased into a front position relative to the housing and is movable to a rear position relative to the housing when the skin sensor element is pressed against an injection site; a needle assembly comprising a hypodermic needle ( 11 ), the hypodermic needle extending outside of the housing when the device is in a needle insertion configuration; a drug delivery mechanism comprising a stored energy source ( 4 ), within the housing; wherein the drug delivery mechanism is released when the skin sensor element is moved from the front position towards the rear position and wherein the stored energy source is arranged to expand along an axis that is offset from an axis of travel of the needle assembly in use; a needle retraction mechanism configured to withdraw the hypodermic needle into the housing when the needle retraction mechanism is released; wherein the needle retraction mechanism is coupled to the skin sensor element such that when the skin sensor element is moved from the rear position towards the front position, and the needle is in the needle insertion position, the needle retraction mechanism is released.

FIELD OF THE INVENTION

The invention relates needle safety mechanisms for injection devices.

BACKGROUND TO THE INVENTION

Needle-based drug delivery devices such as syringes and autoinjectorstypically incorporate a needle safety mechanism to reduce the risk ofaccidental needle stick injuries after drug delivery.

Most of these mechanisms are ‘passive’ in that they deployautomatically, without the user needing to perform any extra actions toactivate them after the drug has been delivered. Passive mechanisms havea clear advantage over ‘active’ systems (where the user needs to deploythe needle safety mechanism after drug delivery as a separate action) inthat they ensure that the used needle is shielded.

These ‘passive’ mechanisms tend to fall into one of two types:‘retracting needle’ type mechanisms and ‘extending cover’ typemechanisms.

Typically in retracting needle type mechanisms the needle isautomatically withdrawn from the patient into the drug delivery deviceat a point dictated by the internal drug delivery mechanism. One issuethat can occur with this approach is withdrawal of the needle beforedrug delivery is complete, resulting in drug not being delivered to thecorrect place in the patient. This is because it is difficult to createa delay between end of drug delivery and needle withdrawal without acomplex ‘lost motion’ mechanism. Manufacturing tolerances prevent needlewithdrawal at the exact point that drug delivery is completed. Any lostmotion mechanism increases the size and complexity of the device.

A second issue with the retracting needle approach is that the drug canleak out of the hole left by the needle in the flesh of the patient ifthe needle is removed from the injection site too quickly. This isbecause the drug has not had sufficient time to be absorbed into thepatient's body tissues, but still exists as a bolus within the patient'sbody. There is a therefore a potential benefit in maintaining the needlein position in the patient for a few seconds after the end of drugdelivery to allow the body to accommodate the drug and reduce the riskof this issue.

In “extending cover” type mechanisms after drug delivery, the needleremains in the patient until the device, and hence the needle, iswithdrawn from the patient, at which point a spring-loaded needle covermoves forwards relative to the needle and device body (as held by theuser), and locks into place, shielding the needle. This approach allowsthe potential for the needle to remain in place in the patient untilwell after the completion of drug delivery, without the costs andcomplexity of a delayed withdrawal mechanism.

However, there is a specific disadvantage of this extending coverapproach when longer needles are used in conjunction with small drugdelivery devices, as the extending needle cover needs to be robustenough once extended to withstand handling and bending forces afterdeployment, so can compromise the small size of the device, and/or notbe sufficiently robust.

Some devices, such as autoinjectors, benefit from being small in orderto render them more portable and less frightening. In particular thisimpacts adrenaline autoinjectors, where there are published studiesindicating that a significant cause of fatalities is lack of carriage ofautoinjectors by users due to the size of the device. There is alsoevidence to suggest that the needle length used in the majority ofcurrent intramuscular autoinjectors is too short, and should be around10 mm longer, at 25 mm inserted depth. There is therefore a need forsmaller devices with longer needles.

The “extending cover” needle safety mechanisms have a greater impact ondevice size as the extended needle length to be covered becomes greater,because it has to extend further from the body of the main device whilstmaintaining high levels of mechanical resistance to bend and breakforces once deployed.

The object of the invention is to provide a needle safety mechanismsuitable for small drug delivery devices with long needles, without thedisadvantages of current retracting needle or extending covermechanisms.

SUMMARY OF THE INVENTION

In a first aspect of the invention, there is provided an automatic drugdelivery device comprising: a housing; a skin sensor element coupled tothe housing and movable relative to the housing, wherein the skin sensorelement is biased into a front position relative to the housing and ismovable to a rear position relative to the housing when the skin sensorelement is pressed against an injection site on a patient; a needleassembly comprising a hypodermic needle, the hypodermic needle extendingoutside of the housing when the device is in a needle insertionconfiguration; a drug delivery mechanism comprising a first storedenergy source within the housing; and a needle retraction mechanismconfigured to withdraw the hypodermic needle into the housing when theneedle retraction mechanism is released; wherein the needle retractionmechanism is coupled to the skin sensor element such that when the skinsensor element is moved from the rear position towards the frontposition, and the needle is in the needle insertion position, the needleretraction mechanism is released.

The device has a needle retracting mechanism which is activated by askin sensor element rather than a drug delivery mechanism. This meansthat the needle is withdrawn into the body of the device when the deviceis moved away from the injection site by the user after the drug hasbeen delivered. In this way, the timing of the needle safety mechanismis controlled by the user, not the drug delivery mechanism, but the useris not required to make an additional action in order to deploy theneedle safety mechanism.

Preferably, the drug delivery mechanism is released when the skin sensorelement is moved from the front position towards the rear position.Advantageously, the stored energy source is arranged to expand along anaxis that is offset from an axis of travel of the needle assembly inuse. The fact that the stored energy source is arranged to expand alongan axis that is offset from an axis of travel of the needle assemblyallows for a compact device to be realised. When a spring or springs areused as the stored energy source, this arrangement does not require touse of very large or powerful springs. The stored energy source mayarranged to expand along an axis that is parallel with, or non-parallelwith, an axis of travel of the needle assembly.

As used herein, front and proximal are used to mean the same end of thedevice, which is the end of the device through which drug is deliveredto a patient. Similarly, rear and distal are used to mean the same endof the device, which is the end of the device opposite to the front endof the device. The term “injection site” as used here, means the area ofa patient through which or to which a drug is to be delivered, such as apatient's thigh, torso or arm.

The drug delivery mechanism may be prevented from being released untilthe skin sensor element is moved from the front position to the rearposition. Preferably, the drug delivery mechanism is released as aconsequence of the skin sensor element being moved from the frontposition to the rear position. Alternatively, the movement of the skinsensor to the rear position may unlock a secondary release mechanism,such as a button.

In operation, the first stored energy source of the drug deliverymechanism may move the hypodermic needle from an initial position to aninsertion position, and when the hypodermic needle is in the insertionposition and the skin sensor element is moved from the rear positiontowards the front position, the hypodermic needle is uncoupled from thefirst stored energy source of the drug delivery mechanism.Alternatively, the first stored energy source of the drug deliverymechanism may move the hypodermic needle from an initial position to aninsertion position, and when the hypodermic needle is in the insertionposition and the skin sensor element is moved from the rear positiontowards the front position, the hypodermic needle may remain coupled tothe first stored energy source but a second stored energy source may bereleased to overcome any force provided by the first stored energysource, to withdraw the needle into the housing.

The drug delivery mechanism may comprise a further stored energy sourcethat, in operation, is released to deliver a drug through the hypodermicneedle. Alternatively, the device may be configured so that the firststored energy source is used both to move the hypodermic needle from aninitial position to an insertion position and to deliver the drugthrough the hypodermic needle.

The needle assembly preferably comprises a drug container containing adrug for injection. The hypodermic needle may be fixed to the drugcontainer. The needle assembly may comprise a plunger rod. The drugdelivery mechanism may be configured to move the needle and drugcontainer to the insertion position and subsequently to move the plungerrod relative to the drug container to deliver the drug.

The drug delivery mechanism may comprise a drive member positionedbetween the first stored energy source and the needle assembly. Thedrive member may comprise a resilient portion that engages the needleassembly to drive the needle to a needle insertion position. Theresilient portion may be moved out of engagement with the needle hub ordrug container when the needle reaches the insertion position and theskin sensor element is moved from the front position to the rearposition, so as to uncouple the first stored energy source from theneedle assembly.

The drug delivery mechanism may prevent release of the needle retractionmechanism until the needle is in the needle insertion position. The drugdeliver mechanism may prevent release of the needle retraction mechanismuntil at least some drug has been delivered through the hypodermicneedle.

The skin sensor may be locked in the forward position after the needleretraction mechanism has been activated, to provide additional distancebetween the tip of the needle and the front of the skin sensor thatcovers it.

The first stored energy source may comprise one or more springs.Alternatively, the stored energy source may comprise a differentresilient element or a compressed gas. The stored energy source maycomprise an electrical energy store.

The needle retraction mechanism may comprise a second stored energysource configured to withdraw the hypodermic needle into the housing.

The second stored energy source may be restrained from retracting theneedle by the first stored energy source. The first stored energy sourcemay transfer energy to the second stored energy source as the needle ismoved to the needle insertion position. In one embodiment, the secondstored energy source is a spring, herein referred to as the needlesafety spring. The needle safety spring may be compressed by the drugdelivery mechanism as the needle is moved to the insertion position.When the hypodermic needle is in the insertion position and the skinsensor element is moved from the rear position towards the frontposition, the hypodermic needle may be uncoupled from the first storedenergy source of the drug delivery mechanism. When the needle isuncoupled from the first stored energy source of the drug deliverymechanism, the needle safety spring can expand to retract the needleinto the housing. As an alternative to a spring, the second storedenergy source may be a gas that is compressed by the drug deliverymechanism as the needle is moved to the insertion position.

The needle safety spring can be held in a compressed condition by aretention feature prior to use of the device, so that it does not impacton the drug container before use, or cause the needle insertion or drugdelivery force to be reduced. The needle safety spring can be uncoupledfrom the retention feature by travel of the skin sensor during use, orby the needle insertion or drug delivery mechanisms during use.

Alternatively, the first stored energy source of the drug deliverymechanism may move the hypodermic needle from an initial position to aninsertion position, and when the hypodermic needle is in the insertionposition and the skin sensor element is moved from the rear positiontowards the front position, the hypodermic needle may remain coupled tothe first stored energy source but a second stored energy source may bereleased to overcome any force provided by the first stored energysource, to withdraw the needle into the housing. In one embodiment thesecond stored energy source is a needle safety spring that is locked ina compressed state until the skin sensor is moved from the rear positiontowards the front position, and the needle is in the needle insertionposition. When the needle safety spring is released it expands andovercomes the force provided by the first stored energy source, towithdraw the needle into the housing. The first stored energy source maybe a spring or springs with a smaller spring constant than the needlesafety spring.

The drug delivery mechanism may be prevented from being released untilthe skin sensor element is moved from the front position to the rearposition. Preferably, the drug delivery mechanism is released as aconsequence of the skin sensor element being moved from the frontposition to the rear position. The drug delivery mechanism may berestrained from release by a coupling between the drug deliverymechanism and the housing. This coupling may be released when the skinsensor is in the rear position. The drive member of the drug deliverymechanism may be restrained relative to a portion of the housing by alocking member engaging a portion of the drive member. The lockingmember may be part of the housing or may be a separate element. Thelocking member may be restrained from moving out of engagement with thedrive member by the skin sensor until the skin sensor is moved to therear position. In the rear position an aperture in the skin sensor or adiscontinuation of a locking surface on the skin sensor, may align withthe locking member, allowing the locking member to move out of theengagement with the drive member.

In one embodiment, the locking member comprises a plurality of balls.The balls are each engaged with a recess in the drive member and in anaperture in an inner portion of the housing to restrain the drugdelivery mechanism from moving relative to the housing. The skin sensorprevents the balls from moving out of the recess in the drive memberuntil it is in the rear position. When the skin sensor is in the rearposition, the first stored energy source urges the balls out of therecess and into an opening in the skin sensor. As an alternative, thelocking member may comprise a plurality of latches on the housing thateach engage a corresponding latch or recess on the drive member. Thelatches may be provided on resilient limbs that are prevented fromflexing to disengage the latches from the drive member by the skinsensor. Only when the skin sensor is in the rear position are thelatches able to disengage from the drive member.

The skin sensor may comprise a plurality of locking surfaces. The skinsensor may comprise a plurality of apertures. The skin sensor may bebiased into a forward position relative to the housing by one or moreskin sensor springs.

The drive member may comprise a resilient portion that deforms todisengage from the needle assembly when the needle assembly is in theinsertion position and the skin sensor is moved out of the rearposition. In one embodiment, the drive member comprises a spring seatand an engagement portion configured to engage the needle assembly,wherein the spring seat and the engagement portion are connected by theresilient portion. The resilient portion may comprise one or moreresilient arms. The drive member may comprise a plurality of springseats and plurality of engagement portions, each spring seat connectedto an engagement portion by one or more resilient arms. The engagementportions may engage a needle hub or the drug container. Alternatively,when the first stored energy source is also used to deliver the drug,the engagement portions may engage a plunger rod. The plunger rod may beconfigured to move in the drug container to deliver the drug through thehypodermic needle.

The resilient arms may be configured to flex in a direction orthogonalto a direction of travel of the needle assembly from an initial positionto the needle insertion position. The resilient arms may be held intension by the first stored energy source as the needle assembly movesto the needle insertion position.

The drive member may comprise at least two resilient arms extending onopposite sides of needle assembly and at least one spring seat connectedto the resilient arms and engaging the stored energy source of the drugdelivery mechanism. In one embodiment, the drive member comprises twopairs of resilient arms, the resilient arms in each pair of resilientarms coupled together by an engagement portion that engages a rear endof a plunger rod.

The needle assembly may comprise a cam surface that engages the drivemember to ensure disengagement of the needle assembly from the drivemember when the one or more resilient arms is allowed to flex. The camsurface may be provided on a needle hub, the drug container or on theplunger rod. A cam surface may be provided to engage with each of theengagement portions of the drive member.

Alternatively, or in addition, cam surface may be provided on each ofthe engagement portions of the drive member.

Each of the cam surfaces on the needle assembly may abut an engagementportion of the drive member at an angle oblique to the direction oftravel of the needle assembly relative to the housing, so that when theone or more resilient arms is allowed to flex, the action of the firststored energy source or the second stored energy source, or both thefirst stored energy source and the second stored energy source, forcesthe needle assembly to disengage from the drive member.

The first stored energy source may be arranged within the housing sothat the needle travels through or past at least a portion of the firststored energy source as it is withdrawn into the housing. This allowsfor a compact device. In one embodiment the first stored energy sourcecomprises first and second drive springs arranged on opposite sides ofthe needle assembly when the needle is in a retracted position. Thesecond stored energy source may be a spring arranged to expand in aspace between the drive springs.

The drug delivery device may be an autoinjector. The autoinjector may beconfigured to be manually held in operation.

The device may comprise a drug for delivery to a patient. The drug maybe a liquid. In one embodiment the drug is epinephrine.

The needle may have an extended length, i.e. the length of the needlethat extends into the injection site, of between 5 mm and 50 mm, andpreferably between 10 mm and 25 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described in detail, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a schematic cross-sectional illustration of a drug deliverydevice in accordance with a first embodiment of the invention, beforeuse:

FIG. 2 is an alternative cross-sectional view of the device of FIG. 1 ;

FIG. 3 is an exploded view of the device of FIGS. 1 and 2 ;

FIG. 4 is a further cross-sectional view of the device of FIG. 1 ,illustrating the locking mechanism restraining the main drive springs,with the cap removed:

FIG. 5 is a still further cross-sectional view of the device of FIG. 1illustrating the locking mechanism restraining the main drive springs;

FIG. 6 shows the device of FIG. 1 with the cap removed;

FIG. 7 is a cross-sectional view of the device of FIG. 6 , with the skinsensor moved to the ‘activated’ position;

FIG. 8 is an alternative cross-sectional view of the device of FIG. 7showing the locking mechanism in the unlocked position;

FIG. 8A is a detailed view of FIG. 8 ;

FIG. 9 is a perspective view of the yoke component;

FIG. 10 is a perspective view of the inner housing;

FIG. 11 is a cross-sectional perspective view of the inner housing;

FIG. 12 is a cross-sectional view of the device of FIG. 8 after theneedle has moved to the ‘inserted’ position

FIG. 13 is an alternative cross-sectional view of the device of FIG. 12showing the drive spring retention mechanism in the unlocked position;

FIG. 14 is a cross-sectional view of the device of FIG. 13 after thedrug seal has ruptured, opening a fluid path between the drug and theneedle;

FIG. 15 is an alternative cross-sectional view of the device of FIG. 14;

FIG. 16 a is an alternative section view of the autoinjector of FIG. 14;

FIG. 16 b shows a detail of FIG. 16 a , with the outer case hidden forclarity;

FIG. 17 shows a similar detail section view of the autoinjector to FIG.16 b , with the outer case hidden for clarity, at a point after the skinsensor has been allowed to move forwards;

FIG. 18 shows a section view of the autoinjector after the cartridge,needle and plunger rod have been pushed back by the needle safetyspring:

FIG. 19 a is a section view of the autoinjector of FIG. 1 after use,with the needle retracted back inside the body;

FIG. 19 b is a detail view of FIG. 19 a;

FIG. 20 a is a schematic illustration of an alternative embodiment inaccordance with the invention, prior to use;

FIG. 20 b shows the embodiment of FIG. 20 a with the skin sensorretracted;

FIG. 20 c shows the embodiment of FIG. 20 b with the needle in aninsertion position;

FIG. 20 d shows the embodiment of FIG. 20 c after the drug has beendelivered;

FIG. 20 e shows the embodiment of FIG. 20 d after withdrawal of the skinsensor from the injection site; and

FIG. 20 f shows the embodiment of FIG. 20 e with the needle retracted.

DETAILED DESCRIPTION

FIG. 1 shows a section view of an example of an autoinjector inaccordance with the present invention. FIG. 2 is an alternative sectionview of the autoinjector of FIG. 1 . FIG. 3 is an exploded view of theautoinjector of FIG. 1

The autoinjector comprises a cartridge 8 that contains a drug 6. Thecartridge is attached to a needle hub 9. A needle 11 is fixed to the hub9. The cartridge 8 is sealed from the needle 11 by a drug seal 10. Theother end of the cartridge 8 is closed by a plunger 5.

The autoinjector has an external housing having an upper housing 1 and alower housing 24. The external housing contains the cartridge 8. In use,the cartridge is moved relative to the external housing to insert theneedle 11 into an injection site, as will be described. Two main drivesprings 4 are provided to drive the cartridge forward to insert theneedle into the injection site and subsequently to move the plunger 5within the cartridge to eject the drug 6. The main drive springs 4 arepositioned between the external housing 1 and a yoke 7. Before use ofthe autoinjector the main drive springs are in a compressed condition,as shown in FIG. 1 . They are held in a compressed condition before usebecause the yoke is restrained from moving relative to the externalhousing. A back portion of the yoke 20 bears on a plunger rod 2. Theplunger rod 2 bears on the plunger 5.

An inner housing 3 is fixed to both the upper external housing 1 and thelower external housing 24.

A skin sensor 15 is provided at a front end of the autoinjector and isslidable coupled to the inner housing 3, and extends within the externalhousing 1. The skin sensor 15 is urged forwards by two skin sensorsprings 14 mounted between the skin sensor and the inner housing 3, butretained on the inner housing 3 by engagement of skin sensor bracingarms 26 with a recess on the inner housing.

A needle safety spring 13 is provided between the needle hub 9 and afront end of the inner housing 3. The needle safety spring is muchweaker than the main drive springs and is initially in an uncompressedcondition.

A cap 16 is coupled to the external housing 1 and covers the front ofthe autoinjector. The cap includes a needle shield portion 12 that ispositioned within the needle safety spring.

The autoinjector also includes a locking mechanism that prevents releaseof the drive springs 4. The components of this mechanism can be seen inFIG. 3 . Four locking balls 17 are retained in corresponding recesses 18in the yoke 7 and holes 28 in the inner housing 3, preventing relativemovement between the yoke and the inner housing.

FIG. 4 is a section view of the autoinjector of FIG. 1 with the cap 16removed, showing the locking balls 17. The locking balls 17 prevent theyoke 7 from being moved forward by the main drive springs 4, by lockingthe yoke 7 to the inner housing 3. The locking balls 17 are retained inposition by the skin sensor 15.

FIG. 5 is an alternative section view of the autoinjector of FIG. 4 ,showing the cap in place.

The various components shown in FIG. 3 may be moulded or otherwiseformed from plastics materials and metals or other materials commonlyused in drug delivery devices.

The operation of the autoinjector shown in FIGS. 1 to 5 will now bedescribed.

FIG. 6 is a section view of the autoinjector of FIG. 1 with the capremoved, ready to use.

To activate the autoinjector, the skin sensor 15 is pressed against aninjection site on a patient. FIG. 7 is a section view of theautoinjector of FIG. 6 with the skin sensor 15 moved backwards,compressing the biasing springs 14.

FIG. 8 is a section view of the autoinjector of FIG. 7 , showing thelocking balls 17 moving out of the locked position in the recesses 18 inthe yoke 7, into an unlocked position that allows the yoke to moverelative to the inner housing. This is possible because holes 19 in theskin sensor 15 have lined up with the locking balls 17 as the skinsensor 15 has been moved backwards. FIG. 8 a is a detail view of FIG. 8showing the holes 19 more clearly.

When the locking balls are moved out of the recesses 18 on the yoke, theyoke is driven forwards relative to the inner housing 3 by the maindrive springs 4. It main drive springs 4 are positioned on oppositesides of the cartridge 8. It can be seen that the drive springs 4 expandalong an axis offset from the axis of the needle. The back of the yoke20 has thrust arms 22 that engage the plunger rod 2 and so moves theplunger rod 2 forwards, and with it the cartridge, needle and hub. Thedrug seal 10 prevents plunger moving within the cartridge and the drug 6from being dispensed through the needle 11 at this stage.

FIG. 9 is a perspective view of the yoke 7 component. It has two thrustfaces 21 to take the load from the two main drive springs 4, and twofurther thrust arms 22 that apply this load to the plunger rod 2, andwhich form part of the back 20 of the yoke 7. Before and during drugdelivery, these thrust arms 22 are held closed by bearing surfaces 27bearing on corresponding bearing surfaces 23 on the inner housing 3, asshown in FIG. 8 .

FIG. 10 is a perspective view of the inner housing 3. The bearingsurfaces 23 of the inner case 3 retain the yoke thrust arms 22, asdescribed. The holes 28 work in combination with the locking balls 17 tolock the yoke 7 in position until the autoinjector is activated.Protrusions 29 are used to lock the inner housing to the upper externalhousing 1. Protrusions 30 are used to lock the inner housing to lowerexternal housing 24. FIG. 11 is a section view of the inner case 3 ofFIG. 10 .

FIG. 12 shows a section view of the autoinjector of FIG. 1 after theneedle 11 has been inserted into the patient. The needle safety spring13 has been compressed by the main drive springs 4 acting on the back 20of the yoke 7, which in turn moves the plunger rod 2 forwards, and withit the cartridge, needle and hub. As described, the drug seal 10prevents the drug 6 from being dispensed through the needle 11 at thisstage.

FIG. 13 is an alternative section view of the autoinjector of FIG. 12 .

Once the cartridge reaches the forward position shown in FIGS. 12 and 13, it cannot move any further forwards. The pressure exerted by the maindrive springs 4 on the plunger 5 through the yoke and plunger rod thencauses the drug seal 10 to deform and rupture on a back end of theneedle 11. Once the drug seal 10 has been ruptured, the drug can bedelivered to the injection site through the needle. The plunger rodmoves relative to the cartridge to dispense the drug until a rear lip onthe plunger rod abuts a rear end of the cartridge 8.

FIG. 14 is a section view of the autoinjector of FIG. 1 after the drug 6has been dispensed. The cartridge 8 has moved fully forwards and thepressure on the drug 6 due to the main drive springs 4 acting on theback 20 of the yoke 7 has caused the drug seal 10 to stretch and bepierced by the back end of the needle 11, allowing the drug 6 to bedispensed through the needle 11 into the patient.

FIG. 15 is an alternative section view of the autoinjector of FIG. 14 .It can be seen that with the cartridge in the fully forward position,the yoke bearing surfaces 27 are no longer supported by inner casebearing surfaces 23, but are supported by the skin sensor bearingsurfaces 25.

Following delivery of the drug, the autoinjector is removed from theinjection site. The autoinjector is constructed so that when the skinsensor 15 is moved forwards again by the skin sensor springs 14 as theautoinjector is removed from the injection site, the needle 11 isretracted back into the inner housing 3

FIG. 16 a is an alternative section view of the autoinjector of FIG. 14. FIG. 16 b shows a detail of FIG. 16 a , with the outer case hidden forclarity. The skin sensor bracing arms 26 constrain the yoke thrust arms22 so that they continue to bear on the plunger rod 2 even though theyare no longer held in place by the inner body bearing surfaces 23. Thisis because the back of the yoke 20 is in contact with the skin sensorbearing surfaces 25. The outer case 1 (not shown) supports the skinsensor bracing arms 26.

FIG. 17 shows a similar detail section view of the autoinjector to FIG.16 b , with the outer case hidden for clarity, at a point after the skinsensor 15 has been allowed to move forwards by the skin sensor springs14 due to the autoinjector being pulled away from the injection surfaceof the patient following drug deliver. It can be seen that the skinsensor bracing arms 26 have moved forwards with the rest of the skinsensor, allowing the yoke thrust arms 22 to move apart, releasing theplunger rod 2 from engagement with the yoke and main drive springs 4.With the cartridge no longer engaged with the main drive springs, theneedle safety spring urges the needle hub 9, and with it the needle 11,cartridge 8 and plunger rod 2, rearwards within the inner housing 3.

FIG. 18 shows a section view of the autoinjector after the cartridge 8,needle 11 and plunger rod 2 have been pushed back by the needle safetyspring 13, having been released from the main drive springs due to themovement of the yoke thrust arms 22. The needle is moved back past themain drive springs 4 until the plunger rod 2 abuts the external housing1. In this position the needle 11 is retracted inside the externalhousing.

FIG. 19 a is a section view of the autoinjector of FIG. 1 after use,with the needle retracted back inside the body. The yoke arms 22 havesprung back to their original position due to the resilience of the yokematerial, causing the back of the yoke 20 to block forward movement ofthe cartridge 8 and needle 11. This prevents the risk of a needle stickinjury due to the needle moving forwards against the needle safetyspring 13, for instance due to inertia during handling of a usedautoinjector. FIG. 19 b is a detail view of FIG. 19 a.

The embodiment described above has main drive springs that expand alongaxes offset from but parallel to the axis of movement of the needleassembly during operation. However it is possible for the drive springor springs, or other stored energy source, to expand along an axisnon-parallel to the direction of travel of the needle. FIGS. 20 a-20 eare a schematic illustration of such a device.

FIG. 20 a is an illustration of an alternative embodiment in accordancewith the invention, prior to use. The device shown in FIG. 20 is anautoinjector comprising a housing 40 containing a cartridge 42containing a drug to be delivered. A hypodermic needle 44 is fixed tothe cartridge. A rear end of the cartridge is closed by a plunger 46. Adrive spring 48 is positioned to drive the plunger 46 through thecartridge 42 to eject the drug through the needle 44. Prior to use, thedrive spring is restrained from acting on the plunger 46 by theengagement of a protrusion 50 on the housing with a drive element 49positioned between the drive spring and the plunger. A skin sensorelement 52 is pivotally connected to the housing 40. The skin sensorelement 52 has an aperture 53, which aligns with an aperture 43 on thehousing, through which the needle can pass in use, as will be described.

FIG. 20 b shows the device of FIG. 20 a with the skin sensor element 52pushed back against the housing 40 when it has been placed on aninjection site. FIG. 20 c shows the device of FIG. 20 b with the needlesubsequently moved to a needle insertion position. The cartridge anddrive mechanism are pivoted within the housing so that the needleextends through apertures 43 and 53 and into the injection site. Aneedle insertion mechanism (not shown) may be used to drive the needleto the needle insertion position shown in FIG. 20 c . The needleinsertion mechanism may be released by the movement of the skin sensoror by other means, such as a user actuated button. Alternatively theneedle insertion may be effected manually.

Movement of the cartridge to the needle insertion position releases theprotrusion 50 from the drive element 49. This releases the drive spring48 to push the plunger 46 through the housing to eject the drug throughthe needle and into the injection site. FIG. 20 d shows the device ofFIG. 20 c after the drug has been ejected.

Following drug ejection, the device is lifted from the injection site.The skin sensor element 52 is then pushed away from the injection siteby a biasing spring (not shown). FIG. 20 e shows the device of FIG. 20 dafter withdrawal of the device from the injection site, with the skinsensor element extending from the housing. Extension of the skin sensorreleases a needle retraction mechanism (not shown) that pulls thecartridge back to its initial position within the housing, whichwithdraws the needle back inside the housing. FIG. 20 f shows the deviceof FIG. 20 e with the needle retracted inside the housing.

It should be clear that the embodiments described are just examples ofdevices in accordance with the invention. Modifications can be made andalternative specific mechanisms used for locking and releasing thecomponents of the device during use. For example, the locking balls 17could be replaced by locking latches on the inner housing. The needlesafety spring may be retained in a compressed condition before use andreleased by movement of a locking latch when the cartridge is movedforwards through the inner housing. Different arrangements for therelative direction of travel of the needle and the expansion of thestored energy source can be foreseen. These and other modificationscould easily be foreseen by a skilled person.

The invention claimed is:
 1. An automatic drug delivery devicecomprising: a housing; a skin sensor element coupled to the housing andmovable relative to the housing, wherein the skin sensor element isbiased into a front position relative to the housing and is movable to arear position relative to the housing when the skin sensor element ispressed against an injection site on a patient; a needle assemblycomprising a hypodermic needle, the hypodermic needle extending outsideof the housing when the hypodermic needle is in a needle insertionposition; a drug delivery mechanism comprising a first stored energysource within the housing and a further stored energy source, wherein,in operation, the first stored energy source is configured to move thehypodermic needle from an initial position to the needle insertionposition and the further stored energy source is releasable to deliver adrug through the hypodermic needle; and a needle retraction mechanismconfigured to withdraw the hypodermic needle into the housing when theneedle retraction mechanism is released, wherein the needle retractionmechanism is coupled to the skin sensor element such that when the skinsensor element is moved from the rear position towards the frontposition, and the hypodermic needle is in the needle insertion position,the needle retraction mechanism is released.
 2. An automatic drugdelivery device according to claim 1, wherein when the hypodermic needleis in the needle insertion position and the skin sensor element is movedfrom the rear position towards the front position, the hypodermic needleor the needle assembly is uncoupled from the first stored energy sourceof the drug delivery mechanism.
 3. An automatic drug delivery deviceaccording to claim 2, wherein the drug delivery mechanism comprises adrive member positioned between the first stored energy source and theneedle assembly, and wherein the drive member comprises a resilientportion and engages the needle assembly to drive the hypodermic needleto the needle insertion position.
 4. An automatic drug delivery deviceaccording to claim 3, wherein the resilient portion is moved out ofengagement with the needle assembly when the hypodermic needle reachesthe needle insertion position and the skin sensor element is moved fromthe rear position to the front position, so as to uncouple the firststored energy source from the needle assembly.
 5. An automatic drugdelivery device according to claim 1, wherein the needle retractionmechanism comprises a second stored energy source configured to withdrawthe hypodermic needle into the housing.
 6. An automatic drug deliverydevice according to claim 5, wherein the second stored energy source isrestrained from retracting the hypodermic needle by the first storedenergy source.
 7. An automatic drug delivery device according to claim5, wherein the first stored energy source transfers energy to the secondstored energy source as the hypodermic needle is moved to the needleinsertion position.
 8. An automatic drug delivery device according toclaim 5, wherein the second stored energy source is a spring which iscompressed by the drug delivery mechanism as the needle assembly ismoved to the needle insertion position.
 9. An automatic drug deliverydevice according to claim 5, wherein the second stored energy source isa spring that is locked in a compressed state until the hypodermicneedle is in the needle insertion position and the skin sensor is movedfrom the rear position towards the front position.
 10. An automatic drugdelivery device according to claim 5, wherein the second stored energysource is a spring held in a compressed condition by a retention featureprior to use of the device, and wherein the second stored energy sourceis uncoupled from the retention feature by travel of the skin sensorduring use, or by a needle insertion mechanism during use, or by thedrug delivery mechanism during use.
 11. An automatic drug deliverydevice according to claim 5, wherein when the hypodermic needle is inthe needle insertion position and the skin sensor element is moved fromthe rear position towards the front position, the hypodermic needleremains coupled to the first stored energy source and the second storedenergy source is released to overcome any force provided by the firststored energy source and withdraw the hypodermic needle into thehousing.
 12. An automatic drug delivery device according to claim 2,wherein the needle retraction mechanism comprises a second stored energysource configured to withdraw the hypodermic needle into the housing,and the uncoupling of the hypodermic needle or the needle assembly fromthe first stored energy source of the drug delivery mechanism releasesthe second stored energy source to withdraw the hypodermic needle intothe housing.
 13. An automatic drug delivery device according to claim 1,wherein the drug delivery mechanism is restrained from release by acoupling between the drug delivery mechanism and the housing, andwherein the coupling is released when the skin sensor element is in therear position.
 14. An automatic drug delivery device according to claim13, wherein a drive component of the drug delivery mechanism isrestrained relative to a portion of the housing by a locking memberengaging a portion of the drive component.
 15. An automatic drugdelivery device according to claim 3, wherein the drive member comprisesa spring seat and an engagement portion configured to engage the needleassembly, wherein the spring seat and the engagement portion areconnected by the resilient portion.
 16. An automatic drug deliverydevice according to claim 15, wherein the resilient portion comprisesone or more resilient arms.
 17. An automatic drug delivery deviceaccording to claim 15, wherein the drive member comprises at least tworesilient arms extending on opposite sides of the needle assembly and atleast one spring seat connected to the resilient arms and engaging thefirst stored energy source of the drug delivery mechanism.
 18. Anautomatic drug delivery device according to claim 1, wherein the firststored energy source is arranged within the housing so that thehypodermic needle travels through or past at least a portion of thefirst stored energy source as the hypodermic needle is withdrawn intothe housing.
 19. An automatic drug delivery device according to claim18, wherein the first stored energy source comprises first and seconddrive springs arranged on opposite sides of the needle assembly when thehypodermic needle is in a retracted position.
 20. An automatic drugdelivery device according to claim 19, wherein the needle retractionmechanism comprises a second stored energy source which is a springarranged to expand in a space between the first and second drivesprings.